1. Field of the Invention.
The present invention relates to liquid level sensors generally and, more particularly, to a novel liquid level sensor that employs a movable magnet with a mirror thereon to make or break a fiber optic light beam.
2. Background Art.
Liquid level sensors are well known devices for detecting the level of any of a number of liquids and include ball-float mechanisms, magnetic-type float gages, torque-tube and force-balance mechanisms, diaphragm-box systems, and manometers, any of which may be used for gaging and/or switching. For example, such a device could be used to measure the level of a liquid in a vessel and shut off a pump when the level reaches a selected value, or it may be used in conjunction with a high- or low-level alarm, or in any other of many known applications for such devices.
All of these conventional systems have the disadvantage of having a critical element in contact with the liquid. This may result in accumulation of foreign matter which interferes with the sensitivity of the element and/or chemical or physical attack by the liquid may also lead to inaccuracies or malflunctions. Some of these devices are relatively complicated and may have linkages, gaskets, and/or bellows which may fail in operation. Many require frequent adjustment or recalibration when in use.
A fairly recent development is a float-type sensor which comprises an annular float freely movable on the exterior of a vertical, hollow tube in response to changes in the level of a liquid in which the tube is positioned. Such sensors are manufactured, for example, by National Magnetic Sensors, Plainville, Connecticut. The float is equipped with a magnet which opens or closes a magnetic reed switch when the level reaches a selected point. Being isolated within the tube, the reed switch cannot be contaminated by enviromental dust, dirt, oil, and humidity. The tube and the float are the only elements of the device which contact the liquid being measured and these may be constructed of virtually any material compatible with the liquid. While these sensors are extremely reliable and the reed switches are rated at up to 100 million cycles, they inherently require that electric leads, albeit at a low electrical potential, must extend to the reed switch. In some cases, this feature means that that type of sensor cannot be used in potentially explosive or flammable environments.
One potential technique for liquid level sensing involves the use of fiber optics, a major advantage thereof being that any electrical elements in the system can be isolated from any hazardous location. Heretofore, however, the use of fiber optics has not proven feasible because attempts to place optical fibers in a vessel and sense the reflection of light from the surface of the liquid have generally been unsuccessful because of fouling of the ends of the optiical fibers.
Accordingly, it is a principal object of the present invention to provide a fiber optic liquid level sensor which isolates the optical fibers from the liquid being sensed.
Another object of the invention is to provide such a sensor that requires no eletrical elements be located in or near the sensor.
An additional object of the invention is to provide such a sensor that may be constructed of any material compatible with the liquid being sensed.
A further object of the invention is to provide such a sensor that may have multiple sense points.
Yet another object of the invention is to provide such a sensor that is rugged and not susceptible to fouling of the critical elements.
Yet an additional object of the invention is to provide such a sensor that is economical to construct and requires no periodic adjustment or recalibration.
Other objects of the invention, as well as particular features and advantages thereof, will, in part, be obvious and will, in part, be apparent from the following description and the accompanying drawing figures.